Molded records, such as conventional audio records or the more recently developed capacitive electronic discs, are manufactured by molding a charge of a thermoplastic composition between a pair of metal parts, referred to as stampers, which have defined in their molding surfaces a mirror image of the pattern desired to be pressed into the molded record. It is conventional to use a charge of the thermoplastic composition which is somewhat larger than that required to exactly mold the record in order to insure that the molded record will not have any voids and that it will be of a uniform density. The excess material in the charge is forced out from between the stampers during the pressing of the record and forms a ring of material around the outer edge of the molded record which is referred to as flash. After the record has been pressed it is necessary to remove the flash from the outer edge of the record and to finish the outer edge to a smooth and precise diameter.
Removing the flash and finishing the outer edge of records is a source of considerable manufacturing problems. One of the major problems which is encountered is breakage of the molded records during trimming. This typically results in sections of the outer portions of the molded records being broken out from the records. Edge cracking is an additional related problem. Microcracks are often formed at the outer edge during trimming which eventually grow into large cracks which destroy the records. A still further problem which is encountered is that the outer edge of the molded records is often left in a roughened condition and not sufficiently round for proper playback. The above-noted problems and other similar problems are especially acute in manufacture of capacitive electronic discs which are heavily loaded with conductive carbon particles.
Many of the manufacturing problems encountered in trimming the flash and finishing the outer edge of records can be attributed to brittleness of the solidified thermoplastic compositions. It is necessary that the molded records be solidified prior to removal from the molding press to prevent warp which will distort the information in the recorded area of the record. However, as the thermoplastic compositions solidifies, the records become relatively brittle and more susceptable to mechanical damage. It has been found, for example, that the carbon-filled capacitive electronic discs are relatively brittle and prone to breakage, edge cracking and are especially difficult to finish.
Various methods and apparatus have been suggested for trimming flash from molded records and for finishing the outer edge of the record. The use of opposing rotary cutter wheels has been suggested, but this apparatus has not proven satisfactory because the rotary wheels tend to pinch the flash and cause a rough, uneven cut. It has also been suggested to use a hot knife to trim the flash, but this has proven to be unsatisfactory as it requires a separate power source for the knife and the edge tends to be distorted. Flusfeder et al. in U.S. Pat. No. 3,412,427, entitled "Apparatus For Manufacturing Disc Records," discloses the use of a pair of overlapping blades for trimming the flash from records. The apparatus which is disclosed is of an overly complex design which makes it difficult to consistently obtain satisfactory results in trimming the flash and no provision is made for finishing the trimmed edge. Pullen et al. in European patent application EP No. 0 044 608 Al, entitled "Improvements Relating To Flash Trimming Of Disk Record," discloses the use of a scissor type cutter which is brought in on a tangent to trim the flash. No provision, however, is made for finishing the trimmed edge. Chambers, et al. in U.S. Pat. No. 4,326,325, entitled "Method And Apparatus For Deflashing Molded Discs," discloses a dual trimming system in which a knife is used to remove the majority of the flash and then a rotary cutter is used to remove the remainder of the flash. The Chambers et al. method provides somewhat better control of flash removal as compared to the other well-known methods, but the outer edge of the disc is left in a considerably roughened condition by the rotary cutter. All of the methods and apparatus heretofore disclosed, while being marginally acceptable for conventional records, were found under production conditions to be unsuitable for trimming flash and finishing the outer edge of capacitive electronic discs.
It would be highly desirable if a method and apparatus could be provided for trimming the flash and finishing the outer edge of records without causing breakage, edge cracking and other such problems.